1. Field of the Invention
The invention relates to microphones, and more particularly to microphone circuits.
2. Description of the Related Art
When audio processing devices are required to record audio signals, the audio processing devices need microphones for converting exterior sound pressures to electric signals. A microphone may be an optional removable component of an audio processing device and inserted to a jack of the audio processing device or removed from the jack of the audio processing device for a user's convenience. Thus, the audio processing device must comprise an interfacing circuit for detecting insertion of the removable microphone in the jack and biasing the removable microphone.
Referring to FIG. 1A, a block diagram of a conventional interface circuit 100 without a microphone plug inserted therein is shown. The interface circuit 100 comprises a jack 102, a resistor 106, a capacitor 108, and an integrated circuit 104. The jack 102 comprises terminals 112 and 116 and a metal sheet 114. The terminal 116 is coupled to a ground voltage source VGND via a node 136. The metal sheet 114 is coupled to a node 144 in the integrated circuit 104 via a node 134. The terminal 112 is coupled to a node 132. The resistor 106 has a resistance ranging between 2.2 kΩ and 4.7 kΩ and is coupled between a high voltage source VDD and the node 132, wherein a voltage of the high voltage source VDD ranges from 2V to 10V. The capacitor 108 has a capacitance ranging between 0.1 μF and 10 μF and is coupled between the node 132 and a node 142 in the integrated circuit 104.
The integrated circuit 105 comprises resistors 122, 124, and 126, an operational amplifier 128, and an analog-to-digital converter 130. The resistor 126 has a high resistance ranging between 200 kΩ and 2 MΩ and is coupled between a high voltage source VDD and the node 144. The resistor 122 is coupled between the node 142 and a negative input terminal of the operational amplifier 128. The resistor 124 is coupled between the negative input terminal of the operational amplifier 128 and an output terminal of the operational amplifier 128. A positive input terminal of the operational amplifier 128 is coupled to a reference voltage source VR. The output terminal of the operational amplifier 128 is further coupled to an input terminal of the analog-to-digital converter 130.
The integrated circuit 104 detects a voltage of the node 144 to determine whether a microphone plug is inserted in the jack 102. When there is no microphone plug inserted in the jack 102 as shown in FIG. 1A, the voltage of the node 144 is at a high level VDD. When there is a microphone plug inserted in the jack 102, the voltage of the node 144 is at a ground level VGND. Referring to FIG. 1B, a block diagram of a conventional interface circuit 150 with a microphone plug 160 inserted therein is shown. The microphone plug 160 comprises a sleeve and a tip respectively coupled to one output terminal of an electret condenser microphone (ECM). When the microphone plug 160 is inserted in the jack 102, the terminal 116 of the jack 102 couples the sleeve of the microphone plug 160 to the ground voltage source VGND. In addition, the sleeve 160 presses the metal sheet 114 of the jack 102 to couple the metal sheet 114 to the ground voltage source VGND. The voltage of the node 144 is therefore lowered to the ground voltage VGND when the microphone plug 160 is inserted in the jack 102.
When the microphone plug 160 is inserted in the jack 102 as shown in FIG. 1B, the terminal 112 of the jack 102 couples the tip of the microphone plug 160 to the node 132. Because a terminal of an electret condenser microphone is coupled to the tip of the plug 160, the high voltage source VDD can therefore bias the electret condenser microphone via the tip of the plug 160. In addition, an AC portion of an output voltage of the electret condenser microphone passes through the capacitor 108 to the node 142. Because the resistors 122 and 124 and the operational amplifier 128 form an amplifier with an inverse configuration, the AC portion of the output voltage of the electret condenser microphone is then amplified by the operational amplifier 128 and delivered to the analog-to-digital converter 130 for analog-to-digital conversion.
The conventional interface circuit 100, however, has a few shortcomings. First, the metal sheet 114 increases the hardware cost of the jack 102. Additionally, the resistor 106 and the capacitor 108 are not integrated into the integrated circuit 104 and must be provided on a printed circuit board, increasing layout cost of the interfacing circuit 100. Moreover, the integrated circuit 104 must have two pins, increasing hardware cost of the integrated circuit 104, wherein a pin couples the node 142 to the capacitor 108 and a pin couples the node 144 to the node 134. Thus, an interfacing circuit for a microphone is therefore provided to reduce hardware cost.